Exfiltration Trench for De-watering discharge

Question

Exfiltration Trench for De-watering discharge

as3usna (Geotechnical)

(OP)

14 Jan 13 12:38

I am designed a shallow exfiltration trench with invert at 4' bgs (below ground surface) with embedded storage chambers due to no allowed off-site discharge and a very small area available. I am having a little difficulty with the entry for Groundwater Elevation and the effects of varying this due to field conditions being somewhat variable. I have tried to be conservative in this. Since the trench invert is 4'bgs and I am using wetted surface and known conductivity (from testing) with an assumed Groundwater elevation of 8' bgs (based upon boring data). My question is, when entering the Groundwater Elevation, does the program account for the differential head or gradient imposed from the bottom of the trench to the Groundwater elevation entered or must this be determined and then entered.

For example, would a assumed groundwater elevation of 6' bgs result in an entry to the program of 6', 2' to bottom of trench, or perhaps 4' to account for a static water elevation within the trench of 2' average depth?

Your prompt response would be appreciated.

psmart (Civil) · Answer 1 · 2013-01-14 14:12:33

When using the saturated hydraulic conductivity, HydroCAD calculates the gradient from the bottom of the pond to the groundwater elevation based on the pond's WSE at each time step. As the WSE increases, the gradient increases.

For complete details see "exfiltration calculations" in HydroCAD help or pages 108-109 of the HydroCAD-10 Owner's Manual.

Peter Smart
HydroCAD Software
www.hydrocad.net

as3usna (Geotechnical) · Answer 2 · 2013-01-14 17:44:37

Thanks, that is what I thought, so when the groundwater elevation in the field rose to the level of the trench invert there was effectively no gradient other than from the stage elevation within the trench. In this case with the trench completely full and with wetted surface exfiltration, a 4' head gradient within the trench to groundwater would have still existed or am I making an overly optimistic conclusion as my original design specified a 4' gradient to an assumed high ground water level of 8' bgs as noted on the boring logs during wet season. Since the trench evidenced failure to handle to design flows, I can only conclude that the information given to me regarding soil conductivity was significantly off or the soil matrix in the area where the trenches were constructed was significantly different from that given in the soil boring logs provided to me by the client. I just wanted make sure I was putting the correct data into the software for the design conditions. Of course if the field characteristics vary significantly from those with which the model was constructed then the model and consequently the "trench" would fail.

Thanks for your read here regarding my forensic conclusions for this design.

psmart (Civil) · Answer 3 · 2013-01-14 18:50:07

The HydroCAD calculation is based on a constant ground water elevation. As of the current HydroCAD-10 release it does not attempt to model groundwater mounding, so the groundwater elevation remains constant (at the preset elevation) for the duration of the routing.

Note that a (manual) increase in the GWE decreases the media thickness, and therefore increases the exfiltration rate according to Darcy's Law. As the GWE approaches the bottom of the pond, the gradient becomes infinite.

Please see my previous references for details.

Peter Smart
HydroCAD Software
www.hydrocad.net

Come Join Us!

Posting Guidelines

Exfiltration Trench for De-watering discharge